Information may be more rapidly processed and transmitted using optical as opposed to electrical signals. Optical signals can be used to enhance the performance of electronic processors. For example, electronic wires interconnecting integrated circuits (ICs) can be replaced with optical interconnects and the information processed with IC driven electro-optic (EO) modulators. Optical signals in fiber optic communications can be encoded on the optical carrier using EO modulators. In both of these processes, nonlinear optical materials with second-order nonlinear optical activity are necessary to effect modulation of the light signal.
Nonlinear optical materials can also be used for frequency conversion of laser light. Such a conversion is desirable in many applications. For example, optical memory media are presently read using 830 nm light from diode lasers. The 830 nm light wavelength limits the spot sizes which can be read and hence the density of data stored on the optical memory media. In fiber optic communications, light wavelengths of 1.3 .mu.m or 1.5 .mu.m are desirable due to the low transmission losses of glass fiber at those wavelengths. However, those wavelengths are too long for detection by Si based detectors. It is desirable to frequency double the 1.3 .mu.m or 1.5 .mu.m wavelengths to 650 nm or 750 nm wavelengths where Si based detectors could be used.
Nonlinear optical materials which have been used in electro-optic devices have in general been inorganic single crystals such as lithium niobate (LiNbO.sub.3) or potassium dihydrogen phosphate (KDP). More recently, nonlinear optical materials based on organic molecules, and in particular polar aromatic organic molecules have been developed.
Organic nonlinear optical materials have a number of potential advantages over inorganic materials. First, organic nonlinear optical materials have higher NLO activity on a molecular basis. Organic crystals of 2-methyl-4-nitroaniline have been shown to have a higher nonlinear optical activity than that of LiNbO.sub.3. Second, the nonlinear optical activity of the organic materials is related to the polarization of the electronic states of the organic molecules, offering the potential of very fast switching times in EO devices. The time response of the organic nonlinear optical system to a light field is on the order of 10 to 100 femtoseconds. In contrast, a large fraction of the second order polarizability in the inorganic crystals in EO applications is due to lattice vibrations in the crystal, slowing the time-response of the materials. In addition, the low dielectric constant of the organic materials (e.g., 2-5 Debye at 1MHz) compared to the inorganic materials (e.g., 30 Debye at 1MHz) enables higher EO modulator frequencies to be achieved for a given power consumption. Third, the organic materials can be easily fabricated into integrated device structures when used in polymer form.
One of the promising and recent approaches to making stable nonlinear optically active organic materials involves forming highly crosslinked networks where polar molecules are polymerized directly into the polymer reagent matrix during the poling process. Eich et al., J. Appl. Phys., 66(7), Oct. 1, 1989, pp 3241-3247, discloses the preparation of nonlinear optically active crosslinked polymer networks from the reaction of epoxides, with and without nonlinear optic dye moleties, and NLO active di- and tri-functional amines, in which the NLO amine is attached to the network by two chemical bonds. Jungbauer et al., Appl. Phys. Lett., 56(26), Jun. 25, 1990, pp 2610-2612, discloses a crosslinked polymer network by reacting a diepoxide with a trifunctional amine in which the NLO active group is attached to the crosslinked polymer network by only one chemical bond. European Patent Application No. 0 474 402 A2 discloses multi-functional chromophore containing polymerizable compounds which are capable of being polymerized into a crosslinked network.
Another approach to making nonlinear optically active organic materials involves side chain liquid crystalline polymers, with the NLO chromophore in the side chain as disclosed in U.S. Pat. Nos. 4,855,376, 4,948,532 and 4,933,112.
Still another approach is disclosed in Allen et. al., J. Appl. Phys., 64(5), Sep. 1, 1988, pp 2583-2589, and involves making nonlinear optically active, single crystal structures of highly conjugated molecules based on substituted dihydropyrazoles.
There is a continuing effort to develop new nonlinear optical polymers with increased nonlinear optical susceptibilities and enhanced stability of nonlinear optical effects.
It is an object of this invention to make polymeric compositions incorporating organic molecular structures which exhibit NLO activity upon orientation. It is an additional object of the present invention that the polymers comprising the NLO molecular structures or chromophores have relatively high glass transition temperatures.
It is still a further object of the invention to provide organic polymeric materials with larger and thermally more stable second order nonlinear optical properties than presently used organic electro-optic materials.